Low pressure rotary compressor type ventilators, for patients who cannot breathe on their own or need assistance in breathing conventionally use either a variable-speed compressor or a constant speed compressor. The former type is exemplified by U.S. Pat. No. 5,694,926 to DeVries et al. and operates by rapidly accelerating from essentially a standstill to produce an inhalation, and then stopping or decelerating to a basal flow level to allow the patient to exhale. The rapid acceleration and braking requires expensive low inertia compressor mechanisms and requires the compressor's drive circuitry to handle very high currents. This in turn requires bulky and expensive power systems and considerable standby battery power when the ventilator is not connected to the commercial power line.
A constant speed compressor such as that shown in U.S. Pat. No. 4,957,107 to Sipin solves this problem but creates another because of the fact that it dumps substantial quantities of air during the exhalation period. This not only causes the compressor to do unnecessary work, but if the inhalation air is oxygenated, it wastes a great deal of oxygen unless the oxygen supply is sealed off during exhalation. Also, such a system as exemplified by U.S. Pat. No. 4,957,107 is inflexible in that air delivery is either on or off, and cannot be varied during a breath to produce complex inspiration pressure and flow curves.
In general, prior art ventilators tend to be relatively large and heavy and allow the patient little mobility; or, if small and light, they tend to be limited in their ventilation capabilities. It is therefore desirable to provide a relatively small and light ventilator system that can be conveniently carried on the back of a wheelchair or even on a person, and that is versatile yet economical in terms of power consumption. Such a device is not currently available.